This doesn't mean the staff are going to run at you with generic leisure wear as you admire the artwork, rather a "front-of-house assistant will nod, smile and, most importantly, make eye-contact," at you when you come in the front door.
Why? To make you feel like a valued and important 'customer' of course... oh, and to stop bad guys.
According to the press, the GAP welcome means that anyone entering the gallery who has a "malevolent purpose," will now immediately "realise they have been recognised," by the chirpy, headset-sporting gap year student on the door.
As international art theives are undoubtedly too clever to walk in through the front door, and Banksy probably doesn't warrant such high levels of security, we can only imagine that "malevolant purpose" is a euphemism for terrorism.
Will a hardened terrorist be put off by the GAP welcome? Let's hope we never have to find out, but if it does maybe they could adopt it on the Tube? Just an idea.